Heavy metals may be found in a number of reductant-containing fluids used or created by industrial processes, particularly those derived from coal, crude oil and some natural gas reserves. Their removal is necessary for the safe and environmentally sound processing of these fluids. For example, emission of heavy metals such as mercury, arsenic, selenium and cadmium from synthesis gases, particularly gases containing hydrogen and carbon oxides derived from coal gasification processes such as Integrated Gasification Combined Cycle (IGCC) processes has become a major environmental concern. Traditional methods for removing mercury, which may exist in either elemental form or as mercuric (i.e. Hg2+) compounds, include trapping it in the ash formed by the gasification process or by using additives in the water-wash stages used to cool and purify the gas streams.
Alternatively the mercury may be trapped using carbon sorbents at low temperature. Such sorbents are limited in their effectiveness and can release mercury during process excursions.
The presence of hydrogen or other reductants in these fluids poses a further complication. Under the conditions at which it is desirable to remove the heavy metals, traditional metal sulphide sorbents can be unstable. For example copper(II)sulphide is reduced in hydrogen containing streams, particularly at temperatures above 150° C. The reaction may be depicted as follows;2CuS+H2→Cu2S+H2S
This reduction process is an undesirable side reaction when it occurs in use, as it may de-stabilise the sorbent and result in the evolution of sulphur containing gas from the bed. Further more it may de-stabilise the Hg already trapped on the sorbent reducing its effectiveness.
US2008/0184884 discloses a process for removing mercury from a reducing gas stream containing hydrogen and/or carbon monoxide and at least one of hydrogen sulphide and/or carbonyl sulphide, by contacting the reducing gas stream with a dispersed copper-containing sorbent at a temperature in the range 25-300° C. Whereas the Cu(II)sulphide sorbent formed appears effective in the presence of free sulphur compounds present in the reducing gas stream under steady-state conditions, it also appears that the sorbent can release the captured mercury under different process conditions. The authors suggest that reduced copper(I)sulphide (Cu2S) is not formed in their process for removing mercury from reducing gas streams.
In view of the variable gas compositions from coal combustion and other processes that produce hydrogen-containing streams, there is a need to provide a high-capacity sorbent for recovering mercury from hydrogen-containing fluids that is more stable than the divalent copper sorbents of the prior art.